We employ "fell" almost exclusively as the past tense of "fall,"
and occasionally as a verb in its own right, meaning "to cut down."
Macduff uses "fell" in a sense that is now rare—as an adjective
meaning "fierce, deadly." King Macbeth, who knows that Macduff is
conspiring to overthrow him, had ordered the murder of Macduff's
wife, children, and servants. This is the "fell swoop": Macduff
likens Macbeth to a "hell-kite" (the kite is a vicious bird of prey
in the falcon family). While Macduff's poultry metaphor has not
worn well, the phrase "one fell swoop" has become a regular part of
the language. Part of its appeal, besides its evocative sound, is
its economic phrasing, which suggests all three meanings of "fell":
Macbeth figuratively "falls" from the sky in a "fierce, deadly"
swoop to "cut down" Macduff's family.